The present invention relates to an anti-entry device for surfaces in which the edge of the surface to be protected is provided with light-reflecting layers which conduct light coming from a source of light to a receiver, said receiver being connected to an alarm which is activated when the light to the receiver is disturbed.
An anti-entry device of the above generally described type is disclosed in West German Patent 23 53 702. The light-reflecting layers in that anti-entry device are disclosed as flat mirror surfaces which are arranged in a sandwich system, for instance between the panes of a window. In this system, the adjustment requirements are minor since the window panes totally reflect light which deviates from the vertical and thus direct it again onto the mirror surfaces.
The expense for adjustment becomes, however, incomparably greater when the anti-entry device is used to secure unglazed surfaces, since the reflected light is reflected by twice the angle of the mirror surface. Therefore even small errors in either the angle between the mirror surfaces or the lateral displacement of the mirror surfaces have a very detrimental effect on the intensity of the light received.